Optical communications is fast becoming the telecommunications industry's standard in terms of reliability and transmission capacity. To this end, optical communications equipment are continually being improved and updated to provide faster, cheaper, and more feature laden alternatives. To provide acceptable performance in optical communications networks, optical amplifiers and optical attenuators amplify or attenuate the optical signal as needed. However, while these amplifiers and attenuators generally provide the required change in optical power to the optical signal, it is standard practice in industry to use automatic output control devices to monitor the optical signal.
Such automatic output control devices, and the systems used in such devices, use a feedback loop to monitor the optical signal strength. If the optical signal strength falls below a certain threshold, a pump laser is activated and enough optical power is inserted into the optical signal to boost its optical power sufficiently to meet predetermined standards. These systems normally use expensive high sped microcontrollers and high speed components to provide fast response times. This approach is seen in the optical amplifier system as disclosed by Yang in U.S. Pat. No. 6,198,571.
In such an approach, a high speed microcontroller is required to perform the multiple calculations and decisions required to provide fast response times. Unfortunately, such an approach is not only complicated but is also quite expensive. The use of A/D (analog-digital) converters which convert the analog optical signal into a digital signal which can be read and used by the microcontroller introduces delays into the response times of the system. Also, such high speed microcontrollers can be quite expensive. Furthermore, the complexity of the software required for these microcontrollers increases the price for the system.
High speed microcontrollers are usually required as Erbium doped fiber amplifiers (EDFAs) may exhibit a transient problem when channels at the amplifier input suddenly increase in either power or number. The gain control for the EFDA must be very fast in order to compensate for the sudden change in input. The digital microcontroller must react within microseconds to be able to provide a stable gain to the other channels in the optical signal. The microcontroller must therefore execute a few iterations to not only detect but compensate for the sudden change. If the reaction time is not fast enough, large overshoots (over-compensation) or undershoots (under-compensation) will occur at the output of the amplifier.
To compensate for such problems, two major approaches are generally followed, both of which use digital circuits. The first is generally known as a feed forward compensation or a digital open loop. In this approach, a step in the gain control circuit is applied in response to a step change in the input power. Essentially, this approach measures the change in input and then calculates the amount by which the gain or output should be adjusted. The pump laser is then adjusted and the output is checked. If the desired level is not achieved, the steps in the loop are repeated. Such an approach, aside from being expensive due to the need for a high speed microcontroller, suffers from the problem of extraneous factors which may affect the performance of the system. Age, temperature effects, noise, and many other factors can degrade the performance of the digital open loop system.
A second approach, called a digital closed loop, compares the desired signal gain with the effective gain during the signal transient. Any difference or error between the two is used in a feedback loop to adjust the setting on the pump laser.
While digital control loops can be fast in terms of response times, they generally require expensive and complex digital components such as dedicated DSPs (digital signal processors) and high speed A/D and D/A converters.
Based on the above, a new approach is therefore needed that will not only provide the required fast response time but will simultaneously provide a solution that is inexpensive. Ideally, such a solution should also provide the flexibility of digital circuits while also providing the required fast response times. It is therefore an object of the present invention to provide alternatives which overcome or at least mitigate the drawbacks of the prior art.